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Summary of Significant Accounting Policies (Fair Value of Financial Instruments) (Details) (USD $) | Dec. 31, 2014 | Dec. 31, 2013
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Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items] |  |
Investments | $ 3,403,822,000 us-gaap_Investments | $ 3,158,312,000 us-gaap_Investments
Total return swap | 0 us-gaap_DerivativeLiabilityFairValueGrossAsset | 1,650,000 us-gaap_DerivativeLiabilityFairValueGrossAsset
Credit facility amount | 150,000,000 us-gaap_LineOfCreditFacilityAmountOutstanding |
Investments [Member] |  |
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items] |  |
Investments | 3,403,822,000 us-gaap_Investments / us-gaap_FinancialInstrumentAxis = us-gaap_InvestmentsMember | 3,158,312,000 us-gaap_Investments / us-gaap_FinancialInstrumentAxis = us-gaap_InvestmentsMember
Interest Rate Swap [Member] |  |
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items] |  |
Interest rate swap agreements, liabilities | 4,025,000 us-gaap_DerivativeLiabilities / us-gaap_FinancialInstrumentAxis = us-gaap_InterestRateSwapMember | 0 us-gaap_DerivativeLiabilities / us-gaap_FinancialInstrumentAxis = us-gaap_InterestRateSwapMember
Secured Debt [Member] |  |
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items] |  |
Financial instrument, fair value | 140,000,000 us-gaap_DebtInstrumentFairValue / us-gaap_FinancialInstrumentAxis = us-gaap_SecuredDebtMember | 140,000,000 us-gaap_DebtInstrumentFairValue / us-gaap_FinancialInstrumentAxis = us-gaap_SecuredDebtMember
Unsecured Debt [Member] |  |
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items] |  |
Financial instrument, fair value | 150,000,000 us-gaap_DebtInstrumentFairValue / us-gaap_FinancialInstrumentAxis = us-gaap_UnsecuredDebtMember | 50,000,000 us-gaap_DebtInstrumentFairValue / us-gaap_FinancialInstrumentAxis = us-gaap_UnsecuredDebtMember
Equity Contract [Member] |  |
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items] |  |
Equity contracts | 194,000 mcy_EquityContractsFinancialInstrumentsLiabilitiesFairValueDisclosure / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_EquityContractMember | 140,000 mcy_EquityContractsFinancialInstrumentsLiabilitiesFairValueDisclosure / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_EquityContractMember
Revolving Credit Facility [Member] |  |
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items] |  |
Credit facility amount | $ 150,000,000 us-gaap_LineOfCreditFacilityAmountOutstanding / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember |

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- Definition Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=28364263&loc=d3e13433-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=28364263&loc=d3e13476-108611 + Details
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- Definition Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=28364263&loc=d3e13433-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=28364263&loc=d3e13495-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=20225523&loc=SL20225862-175312 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=28370219&loc=SL20226008-175313 + Details
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- Definition Fair value of asset associated with financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=28370219&loc=SL20226003-175313 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=20225523&loc=SL20225862-175312 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (a),(c) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624163-113959 + Details
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